1. Field of the Invention
The present invention relates to a method of forming electrodes at the end surfaces of chip array resistors, and more particularly, to a technology of utilizing sputtering deposition with metal mask to form electrodes at the end surfaces of chip resistors without restriction of a punch-through wafer use so that not only producing capability can be promoted but also the quality of the electrodes at the end surfaces of chip array resistors would be maintained effectively.
2. Description of the Prior Art
Generally speaking, forming electrodes at the end surface of chip array resistors is performed in a conventional method by means of coating a metallic gel first, then followed by sintering or baking. However, during the manufacturing process for chip array resistors, frequent sintering under high temperature (in the steps of making thick film for chip array resistors) causes the variation of value of resistance in the resistors after adjusted through a laser trimming. In general, if standard deviation of a resistor's resistance is increased, it results in the decrease of yield rate of production. Moreover, if a metallic gel of baking type is used, the electrodes would be formed on the wafer by deposition of metallic gel whose adhesive force is weaker than that by sintering the metallic gel. No matter what conventional method is used, the standard process would be peeling a wafer in strips and then employing it to form the electrodes either by soaking or by coating. Such processes result in the necessity of employing high cost but unsophisticated punch-through wafer whose size can not be enlarged because of shrinkage caused by sintering. As a result, manufacturing efficiency, product yield rate and quality can not be improved.
The conventional method of forming chip resistance of electrodes at end surfaces of array resistors by sintering after applying an electrically conducting gel with soaking or coating includes the following steps:
1. Printing a punch-through wafer as shown in FIG. 8; PA1 2. Peeling the wafer to form strip-shaped arrays as shown in FIG. 9; PA1 3. Soaking or coating the strip-shaped wafer on the first end of surface (upper Fig.) and then on the second end surface (lower Fig.) as shown in FIG. 10 and 11; PA1 4. Drying the wafer by sintering after being accomplished the step of soaking or coating as shown in FIG. 12; PA1 5. Separating the wafer in pieces as shown in FIG. 13. PA1 A. The Pre-punched holes on the ceramic base will change their size due to shrinking during sintering process. As a result, only specific range of sizes are useful for the manufacturers. For example, the maximum size of ceramic base that suppliers can furnish is 60 mm.times.45 mm which, in general, is ranked per .+-.1 mm for each class. Presently a conventional ceramic base is ranked in 40.about.50 classes so that a manufacturer has to take at least 10.about.20 different classes of the ceramic base to make up about 429 pieces of 1206 chip array resistors. On the contrary, a 3 square inch commercial base may be used to make more than 1200 pieces of chip array resistors. Difference of productivity between the two cases is so obvious and will be much more significant as the size of the base is now shifting to 4 inch or 4.5 inch larger than ever. PA1 B. As the number of classes of punch-through base ranges up so many from 10 to 20, the mesh patterns used to print the base also have to be increased correspondingly, which entails difficulty for centralization. PA1 C. As the difference between the size of adjacent base classes is so small that it is only .+-.0.1 mm, the variation of design of resistance patterns or other printing pattern are restricted accordingly, which results in wasting available spaces.
1) Features of Soaking or Coating
A punch-through wafer is required in forming electrodes at the end surfaces of chip array resistors, it is performed by opening a hole on a ceramic base mold for isolating a plurality of electrode terminals from one another so that electrical connection between adjacent electrode terminals with conducting metallic gel may be prevented. As the finished wafer in complete figure is employed, the electrodes at the end terminals of the chip array resistors may be easily formed after soaking or coating the metallic gel on the end surfaces of the strip-shaped wafer. Such a conventional method of forming electrodes has merits that it is quite appreciated by most of manufacturers in this field because it is technically simple and compatible to other related technology which they have been well experienced.
2) Disadvantages of Soaking or Coating
In spite of its simplicity in making process, a punch-through-base method which requires forming chip array resistors is still with some restrictions in designing and printing, namely:
Furthermore, soaking or coating process itself includes several disadvantages. The metallic conductive gel is exposed to the atmosphere during the procedure which causes the change of material viscosity with respect to the time elapsed. In addition, during the drying step arranged to be accomplished before sintering, the tools and equipment for drying and sintering are not allowed to contact the coated surface. Consequently it is difficult to make and handle those tools and equipment.
However, soaking or coating the end surface of chip array resistors is performed after the step of laser trimming of resistance value, value of the product resistance will vary according to the amount of heat applied to the end surfaces of the chip array resistors during sintering. The longer times the product is sintered, the greater the product resistance varies. Accordingly, quality of the product is degraded if it is impossible to control the resistance accurately.
In order to improve the shortcoming concerning the method of forming electrodes at the end surfaces of chip array resistors, through a long term hard study and experiments, the applicant has developed a novel method of forming electrodes at the end surfaces of chip array resistors and now is intended to disclose it hereafter.